Horizontal vortex ring motion in linearly stratified media

Laminar vortex rings propagating horizontally in linearly stratified media were investigated in a water tank using the laser-induced fluorescence flow visualization technique. The vortex rings were observed to develop an asymmetry where the upper part of the ring became larger in the cross-sectional area than the lower part. The asymmetry was a result of the instability of the interface on the upper portion of the ring; the upper half of the vortex was heavier than its immediate surroundings, whereas the lower half was lighter. When the circulation based internal Froude number was smaller than 1.3, the vortex rings retained their overall geometry and continually decreased in size by leaving significant amounts of dyed fluid behind in their wake. At Froude numbers greater than 3.2, the asymmetry was more pronounced, both in the relative position of the upper and lower cores as well as their sizes, and the overall vertical extent of the rings did not decrease until some time after the vortex formation. These observations led to a two-level classification of the collapse process. The collapse time and distance increased with Froude number at any Reynolds number. For a given Froude number and discharge time, increasing the Reynolds number delayed the vortex ring collapse. The normalized collapse time and distance for the lower Froude number vortex rings in the present study were found to scale with the composite parameter ReFr2/3.